In many electronics implementations mechanical relays are used. There has long been a desire to replace the mechanical relays with solid state circuits or switches, that may be integrated in silicon components. Such circuits have a number of advantages compared to the conventional mechanical relays:
they normally take up less space on the circuit board PA1 they enable a higher degree of integration and thereby also a higher degree of flexibility. PA1 they are normally able to switch from off-state to on-state and vice versa much faster than mechanical relays. PA1 providing a first electronic component, PA1 providing at least one additional transistor, each additional transistor having a specified nominal conductance, and PA1 controlling the conductance of said electronic circuit by means of an electronic adjusting means.
A major drawback with designing electronic system using solid state switches is that their resistance in the on state, or on-resistance, is relatively high compared to mechanical relays. Further, for several applications, the resistance in the on-state has to have an exact value, which can be hard to reach using a solid state switch solution.
In some applications two switches are used together, for example, to connect or disconnect the signal loop between a subscriber and a line circuit in a telephone network. In this case it is important to match the switches to substantially the same resistance, as differences in on-resistance between them will result in deterioration of the longitudinal balance for the signal loop, making the network more sensitive to common mode noise. To keep the longitudinal balance for the signal loop at an acceptable level, the difference in resistance between the two switches should be kept below 1.OMEGA.. A number of switches may be used together in a line circuit, for example, for testing purposes. These switches should also be matched to each other as perfectly as possible. The switches inside the signal loop must also work bidirectionally, that is, they must be able to conduct current in both direction and handle both negative and positive high voltages.
The use of two mirrored transistors, for example, in differential amplifiers or line drivers, also requires exact tuning of the mirrored transistors, to minimize the error introduced in the differential signal by the amplifier or driver itself. It is then important that the output characteristics of the transistors are well tuned both in the linear region (where channel-conductance and on-resistance are defined) and in the saturation region (where the trans-conductance is defined).
Further, because of the requirements on high linearity, even for low voltage drops across the switch is low, it is feasible to use field effect transistors (FETs) rather than bipolar solutions to implement the switch. The linearity is needed to mask out signal distortion.
Field Effect Transistors are unipolar, multielectrode semiconductors, comprising four regions, commonly referred to as ground, source, drain and body. Normally, the body region is connected together with the source region. Current may flow in conducting channels between the source and the drain, and is modulated by an electric field applied at the gate. Application of a suitable bias across the transistor causes charge carriers to flow from the source to the drain of the transistor, that is, the current is controlled by the difference between the gate voltage and the source/drain voltage.
The inaccuracy of a single transistor is determined by variations in the manufacturing process and in the properties of the material used. The inaccuracy of the conductance of a transistor is approximately 10%. The difference in on-resistance between two switches used in a pair should be kept below 1.OMEGA.. Normally, therefore, transistors with an on-resistance up to 10.OMEGA. are used in such applications.
To reduce the inaccuracy in the on-resistance of a switch, the absolute value of the on-resistance is kept low. For example when used to connect and break the signal loop in a telephone network, an on-resistance lower than 20.OMEGA., or even 10.OMEGA. is required to fulfil the requirements of a mismatch lower than 1.OMEGA.. Also, the performance at high voltages must be very good, typically for breakdown voltages higher than 300V for the switch. The requirements on on-resistance and high breakdown voltage together makes a switch based on FETs quite spacious.
A common transistor switch for analogue applications may also comprise two field effect transistors of opposite channel type, connected in parallel. The respective drains and sources of the two transistors are tied together to become the switch terminal, while the gates of the transistors are used to control the on/off action. Essentially, the n channel transistor is on for positive gate-to-source voltages and off for negative gate-to-source voltages (vice versa for the p channel transistor). The on characteristic for such a transistor switch is then sensitive to variations in the conductance of both the n and the p channel transistor.
To fulfil the above requirements on good linearity, high breakdown voltage, and low mismatch of the on-resistance of the switches, and also achieve a low mismatch of the conductance of transistors, currently the devices are overdimensioned, to keep the total on-resistance lower than what is really necessary.